1. Field of the Invention
The present invention relates generally to nuclear quadrupole resonance, and more particularly to stochastic nuclear quadrupole resonance using random or pseudo-random excitation.
2. Description of the Related Art
Nuclear quadrupole resonance (NQR) is a technique for analyzing species having quadrupolar nuclei (spin quantum number I.gtoreq.1, e.g., .sup.14 N, .sup.35,37 Cl, .sup.39 K, etc.). In a typical NQR technique, a sample is excited with radio frequency (rf) radiation, to induce a precession in nuclei by irradiating the nuclei with an excitation frequency corresponding to the resonance frequency of the nuclei. As with better-known nuclear magnetic resonance (NMR), the location of a resonance frequency in a sample depends on the specific chemical environment around a resonant nucleus.
In addition to the location of resonant frequency peaks, the width of resonant frequency peaks can provide information about the material being analyzed. Accordingly, it may be desirable to monitor the widths (typically the full width at half maximum, FWHM) of NQR peaks in various materials.
As shown in FIG. 1, a typical excitation pulse used in NQR detection of a typical 1 cm.sup.3 specimen has a short duration (typically on the order of 0.01-0.1 ms) and a large amplitude (typically on the order of 200 V.sub.r.m.s, corresponding to 800 W at 50 .OMEGA.). This type of excitation pulse results in a broad excitation frequency distribution. However, the higher amplitude of this excitation pulse will dramatically increase the peak power requirement of the excitation source, since the peak power is proportional to the square of the peak voltage. Reducing the power requirements of NQR excitation has been a continuing concern in the art. See U.S. Pat. No. 5,233,300, issued Aug. 3, 1993 to Buess et al., for "DETECTION OF EXPLOSIVE AND NARCOTICS BY LOW POWER LARGE SAMPLE VOLUME NUCLEAR QUADRUPOLE RESONANCE (NQR)", incorporated by reference herein, in its entirety for all purposes.